Reference picture list handling

ABSTRACT

An encoder is configured to encode a representation of a current picture of a video stream of multiple pictures. The encoder is further configured to encode, for each of a plurality of reference pictures included in a buffer description for the current picture, a respective one-bit flag according to one of two available values for the one-bit flag. The two available values for the one-bit flag include a first value explicitly indicating to a decoder to include the reference picture in a reference picture list for decoding the current picture. The two available values for the one-bit flag further include a second value explicitly indicating to the decoder not to include the reference picture in the reference picture list for decoding the current picture. The encoder is further configured to output the representation of the current picture and the one-bit flags.

REFERENCE PICTURE LIST HANDLING

This application is a continuation of prior U.S. patent application Ser.No. 13/704,227 filed on 13 Dec. 2012, which was the U.S. National Stageof International Application No. PCT/SE2012/051030 filed on 27 Sep.2012, which claims the benefit of U.S. Provisional Application Ser. No.61/540,628 filed on 29 Sep. 2011, the disclosures of all of which areeach expressly incorporated by reference herein in their entirety.

TECHNICAL FIELD

The present embodiments generally relates to video encoding anddecoding, and in particular to reference picture handling in such videoencoding and decoding.

BACKGROUND

High Efficiency Video Coding (HEVC) is a new video coding standardcurrently being developed in Joint Collaborative Team-Video Coding(JCT-VC). JCT-VC is a collaborative project between Moving PictureExperts Group (MPEG) and International TelecommunicationUnion-Telecommunication Standardization Sector (ITU-T). An HEVC Model(HM) has been defined that includes a number of new tools and isconsiderably more efficient than H.264/Advanced Video Coding (AVC).

A picture in HEVC is partitioned into one or more slices, where eachslice is an independently decodable segment of the picture. This meansthat if a slice is missing, for instance got lost during transmission,the other slices of that picture can still be decoded correctly. Inorder to make slices independent, they do not depend on each other. Nobitstream element of another slice of the same picture is required fordecoding any element of a particular slice.

Each slice contains a slice header which independently provides allrequired data for the slice to be independently decodable. One exampleof a data element present in the slice header is the slice address,which is used for the decoder to know the spatial location of the slice.Another example is the slice quantization delta which is used by thedecoder to know what quantization parameter to use for the start of theslice. There are many more data elements in the slice header.

HEVC also has mechanisms for handling reference pictures, which arepreviously decoded pictures to be used for decoding of a currentpicture. A reference picture in HEVC is a picture in the decoded picturebuffer (DPB) that is available for reference by being marked “used forreference The pictures to be used as reference pictures are included inreference picture lists, which for HEVC is similar to the referencepicture list in H.264. The reference picture lists are then used in thedecoding process of the current slice in the current picture.

HEVC also defines a temporal_id for each picture, corresponding to thetemporal layer that the picture belongs to. Temporal layers are orderedand are used for temporal scalability where higher temporal layers canbe removed without affecting the decoding of lower temporal layers. Thatmeans that if temporal layer A is higher than temporal layer B, apicture belonging to temporal layer A can use a picture from temporallayer B for prediction but a picture belonging to temporal layer B cannot use a picture from temporal layer A for prediction.

In HEVC, it is proposed to use absolute signaling of reference picturesinstead of signaling reference picture modifications in a relative wayas in previous standards, e.g. H.264. The absolute signaling is realizedby signaling what reference pictures to keep to the decoder in a BufferDescription for each picture explicitly or by signaling them through areference to a Sequence Parameter Set (SPS). The Buffer Description isalso referred to as Reference Picture Set (RPS).

Picture Order Count (POC) is used in HEVC to define the display order ofpictures and also to identify reference pictures. In the first drafts ofHEVC, not only POC was signaled for each reference picture in a BufferDescription but also temporal_id. The values of POC and temporal_id inthe Buffer Description must be identical to the values of POC andtemporal_id signaled in the slice header of the reference picture towhich it is referring. Temporal_id is used during the Buffer Descriptiondecoding process for reference pictures that are included in the BufferDescription but not available in the Decoded Picture Buffer (containingdecoded pictures) in order to deduce if a picture that is not availableor present in the decoding picture buffer has been unintentionally lostor correctly removed. If the reference picture in a Buffer Descriptionhas higher temporal_id than the temporal_id of the current picture it isdeemed correctly removed and the decoding process can continue,otherwise it is deemed unintentionally lost and the current picture maynot be correctly decodable.

It can be noted that the process of deducing whether a missing picturehas been unintentionally lost or correctly removed is independent of theactual Buffer Description decoding process and could be performed beforeor after the Buffer Description decoding process.

Temporal_id is also used in the reference picture list constructionprocess. Reference pictures that belongs to higher temporal layers thanthe temporal layer of the current picture are not included in referencepicture lists of the current picture.

A schematic illustration of the decoding process at a high level asproposed in HEVC can be seen in FIG. 1.

SUMMARY

The embodiments of the present invention provide an efficient solutionfor reference picture handling.

As stated above, in the previous drafts of HEVC, the temporal_id wassignaled for each picture in each Buffer Description using up to threebits, depending on the number of temporal layers. For pictures that arenot available in the DPB (Decoded Picture Buffer) the temporal_id fromthe Buffer Description was used to deduce if the picture is correctlyremoved or unintentionally lost.

When temporal_id is signaled with more than one bit there is aredundancy. For example, when decoding a picture in layer 0 it does notmatter if a certain reference picture belongs to temporal layer 1 or 2to deduce if the picture is correctly removed or unintentionally lost.The only important information is whether it is in a higher layer ornot, since that is the information needed to deduce if the picture iscorrectly removed or unintentionally lost.

In accordance with embodiments of the present invention, a one bit flagset to explicitly indicate to a decoder whether or not the referencepicture should be included in a list of reference pictures is provided.

Thus, according to a first aspect of embodiments of the presentinvention, a method of encoding a representation of a current picture ofa video stream of multiple pictures using reference pictures isprovided. In the method, a one-bit flag associated with a referencepicture in a buffer description for the current picture is encoded,wherein the one-bit flag is set to explicitly indicate to a decoderwhether or not the reference picture should be included in a list ofreference pictures. The list of reference pictures is indicating whichpictures that can be used for references when decoding the currentpicture.

According to a second aspect of embodiments of the present invention, amethod of decoding a representation of a current picture of a videostream of multiple pictures using reference pictures is provided. In themethod, a one-bit flag associated with a reference picture in a bufferdescription for the current picture is decoded. The one-bit flagexplicitly indicates to the decoder whether or not the reference pictureshould be included in a list of reference pictures indicating whichpictures that can be used for references when decoding the currentpicture. The method further comprises the step of using the one-bit flagto determine if the reference picture should be included in the list ofreference pictures.

According to a third aspect of embodiments of the present invention, anencoder for encoding a representation of a current picture of a videostream of multiple pictures using reference pictures is provided. Theencoder is configured to encode a one-bit flag associated with areference picture in a buffer description for the current picture andthe encoder comprises a processor configured to set the one-bit flag toexplicitly indicate to a decoder whether or not the reference pictureshould be included in a list of reference pictures. The list ofreference pictures is indicating which pictures that can be used forreferences when decoding the current picture.

According to a fourth aspect of embodiments of the present invention, adecoder for decoding a representation of a current picture of a videostream of multiple pictures using reference pictures is provided. Thedecoder is configured to decode a one-bit flag associated with areference picture in a buffer description for the current picture. Theone-bit flag explicitly indicates to the decoder whether or not thereference picture should be included in a list of reference pictures,wherein the list of reference pictures indicates which pictures that canbe used for references when decoding the current picture. Further, thedecoder comprises a processor configured to use the one-bit flag todetermine if the reference picture should be included in the list ofreference pictures.

According to a fifth aspect, an encoder for encoding a representation ofa current picture of a video stream of multiple pictures using referencepictures is provided. The encoder is configured to encode a one-bit flagassociated with a reference picture in a buffer description for thecurrent picture. The encoder comprises a processor configured to set aone-bit flag according to a comparison between the layer identity ofeach reference picture and the layer identity of the current picturesuch that the flag indicates that a reference picture is to be includedin the reference picture list if the layer identity of that referencepicture is equal to or lower than the layer identity of the currentpicture and such that the flag indicates that the reference picture isexcluded from the reference picture list if the layer identity of thereference picture is higher than the layer identity of the currentpicture.

According to a sixth aspect a decoder for decoding a representation of acurrent picture of a video stream of multiple pictures using referencepictures is provided. The decoder is configured to decode a one-bit flagassociated with a reference picture in a buffer description for thecurrent picture. The decoder comprises a processor configured to checkthe one-bit flag for each reference picture in the buffer descriptionwhere the corresponding reference picture in a decoded picture buffer ismissing and if the one-bit flag is indicating that the reference pictureis included in the reference picture list, the processor is configuredto conclude that the picture is unintentionally lost.

An advantage with the embodiments of the present invention is that giventhat three bits are spent for each picture in the Buffer description andthat can be reduced to one bit, a bit-saving of 2*N bits per slice canbe achieved if explicit signaling of Buffer Descriptions is used, whereN is the number of reference pictures described in the BufferDescription. Hence, embodiments of the present invention provide asolution where redundancy is removed for signaling whether pictures inthe Buffer Description strictly have to be present in the DPB or not.That is, the information about whether a reference picture has beencorrectly removed or unintentionally lost is indicated by at most onebit instead of up to three bits.

An advantage with the embodiments is that an encoder can choose toexclude certain pictures that belong to the same or lower temporal layerfrom the reference picture lists by setting the value of the flag tozero, which might remove the need for signaling reference picture listmodifications, thus reducing the bitrate and/or reduce the number ofbits needed for signaling reference picture index.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the decoding process at a highlevel as proposed in HEVC according to prior art.

FIG. 2 is a flowchart illustrating the methods in an encoder and in adecoder according to the embodiments of the present invention.

FIGS. 3 and 4 illustrate the encoder and the decoder according to theembodiments of the present invention.

DETAILED DESCRIPTION

The embodiments relate to handling of reference picture lists (alsoreferred to as list of reference pictures) which describes whichpictures to be used for decoding a current picture. In the embodiments,the encoder indicates explicitly by using a one-bit flag (having twopossible values) to a decoder whether or not a reference picture shouldbe included in a reference picture list, wherein the reference picturelist indicates which pictures that can be used for reference whendecoding the current picture.

Accordingly, the reference picture list is a list of pictures that canbe used for reference when decoding a current picture and the referencepicture list is stored at the decoder. The pictures are defined by POC.However, the pictures included in the list are not required to be usedfor reference but pictures not included in the list are not allowed beused for reference when decoding the current picture.

A buffer description, also referred to as Reference Picture Set (RPS),is signalled from the encoder to the decoder in the bitstream and thebuffer description comprises reference pictures and a one-bit flag foreach reference picture. The one-bit flag indicates explicitly for eachreference picture in the Buffer Description whether it should beincluded in the reference picture list.

In a method performed by an encoder according to a first aspect aone-bit flag is signaled, wherein the one-bit flag is associated with areference picture in a buffer description for the current picture. Thisis illustrated in FIG. 2 wherein the one-bit flag 220 is set 200 toexplicitly indicate to a decoder whether or not the reference pictureshould be included in a list of reference pictures.

According to a one embodiment, the one-bit flag is set 201 a such thatreference pictures from higher layers compared to the layer of thecurrent picture are not included in the list. Thus, a processor of theencoder is configured to always set the one-bit flag to explicitlyindicate that the reference picture should not be included in thereference picture list if the layer identity of the reference picture ishigher than the current picture. In this embodiment, it is determined200 if the layer identity of the reference picture is higher than thelayer identity of the current picture, and the one-bit flag is set 201 asuch that it explicitly indicates that the reference picture should notbe included in the reference picture list if the layer identity of thereference picture is higher than the layer identity of the currentpicture.

According to a further embodiment, the one-bit flag is set 201 b suchthat it explicitly indicates that the reference picture should beincluded in the reference picture list if at least the layer identity ofthe reference picture is equal to or lower than the current picture.

That can be done by setting 201 b the one-bit flag to “1” to explicitlyindicate that one reference picture should be included in the referencepicture list if at least the layer identity of the reference picture isequal to or lower than the current picture and setting 201 a the one-bitflag to “0” to explicitly indicate that one reference picture should notbe included in the reference picture list if at least the layer identityof the reference picture is higher than the current picture.

According to a further embodiment, the one-bit flag is set 201 c suchthat it explicitly indicates that the reference picture should beincluded in the reference picture list if the layer identity of thereference picture is equal to or lower than the current picture and ifanother condition is fulfilled. That implies that the reference pictureshould never be allowed to be included if the layer identity is higherthan the layer identity of the current picture but if the layer identityof the reference picture is lower or equal to the layer identifier ofthe current picture, the encoder can choose according to anothercondition if the reference picture should be included and set the flagaccordingly. An example of such another condition is to exclude pictureswith a (base) quantization parameter higher than a specific value.

The one-bit flag 220 associated with a reference picture in a bufferdescription 210 for the current picture is encoded 202 and sent to thedecoder. The one-bit flag 220 can be encoded in a slice header or in aparameter set such as a sequence parameter set or a picture parameterset. The sequence parameter set contains information for more than onepicture.

With further reference to FIG. 2, a method of decoding a representationof a current picture of a video stream of multiple pictures usingreference pictures is provided. A one-bit flag associated with areference picture in a buffer description for the current picture isreceived from the encoder and then decoded 203. The decoder may beconfigured to decode the one-bit flag in a slice header or in aparameter set such as a sequence parameter set.

The one-bit flag explicitly indicates to the decoder whether or not thereference picture should be included in a list of reference picturesindicating which pictures that can be used for references when decodingthe current picture. The one-bit flag is used 204 to determine whetheror not the reference picture should be included in the list of referencepictures.

According to an embodiment, one value of the one-bit flag explicitlyindicates that the reference picture should not be included in thereference picture list when the layer identity of the reference pictureis higher than the current picture. According to another embodiment, onevalue of the one-bit flag explicitly indicates that one referencepicture should be included in the reference picture list if the layeridentity of the reference picture is equal to or lower than the currentpicture.

These embodiments can be exemplified by that the one-bit flag set to “1”explicitly indicates that one reference picture should be included inthe reference picture list and that the one-bit flag set to “0”explicitly indicates that one reference picture should not be includedin the reference picture list.

An encoder 300 according to another aspect of the embodiments asillustrated in FIG. 3 is configured to create and signal a bufferdescription wherein the one-bit flag 220 is set for each referencepicture in the buffer description to a value as decided by the encoder300. That is, the encoder is configured to encode a one-bit flag 220associated with a reference picture in a buffer description 210 for thecurrent picture, wherein the one-bit flag is set to explicitly indicateto a decoder whether the reference picture should be included in a listof reference pictures indicating which pictures that can be used fordecoding the current picture. The encoder 300 comprises a processor 302configured to create the buffer description and to set the one-bit flag220 such that it explicitly indicates whether the reference pictureshould be included in a list of reference pictures or not. The encoder300 may also comprise a memory 301 for storing picture relatedinformation such as information relating to reference pictures and layerinformation. Furthermore, the encoder 300 may be configured to encodethe one-bit flag 220 in a slice header or in a parameter set such as asequence parameter set.

According to an embodiment, the one-bit flag 220 is set such thatreference pictures from higher layers compared to the current pictureare not included in the list. Thus, the encoder is restricted to alwaysset the value of the one-bit flag so that a reference picture from ahigher temporal layer compared to the layer of the current picture isnot included in the reference picture list. That implies that theprocessor 302 is further configured to determine if the layer identityof the reference picture is higher than the layer identity of thecurrent picture and to set the one-bit flag such that it explicitlyindicates that the reference picture should not be included in thereference picture list if the layer identity of the reference picture ishigher than the layer identity of the current picture according to thisembodiment.

The processor 302 is further configured to set the one-bit flag suchthat it explicitly indicates that the reference picture should beincluded in the reference picture list if at least the layer identity ofthe reference picture is equal to or lower than the current picture.

According to a further embodiment, the processor 302 is configured toset the one-bit flag to “1” to explicitly indicate that one referencepicture should be included in the reference picture list and to set theone-bit flag to “0” to explicitly indicate that one reference pictureshould not be included in the reference picture list.

As mentioned above, the reference picture should never be allowed to beincluded if the layer identity is higher than the layer identity of thecurrent picture. If the layer identity of the reference picture is loweror equal to the layer identifier of the current picture, the encoder canchoose if the reference picture should be included by setting the flagaccordingly. In this case, the processor 302 is further configured toset the one-bit flag such that it explicitly indicates that thereference picture should be included in the reference picture list ifthe layer identity of the reference picture is equal to or lower thanthe current picture and if another condition is fulfilled.

The decoder uses the information provided by the one-bit flag 220 tocreate the reference picture list 230. Hence the decoder which performsin accordance with the above described embodiments is configured todecode the slice header. The Buffer Description that includes theone-bit flag 220 for each reference picture in the Buffer Description isdecoded from syntax elements in the slice header and/or possibly fromthe active Sequence Parameter Set. Picture marking is then performedusing the Buffer Description.

According to a further aspect a decoder for decoding a representation ofa current picture of a video stream of multiple pictures using referencepictures is provided as illustrated in FIG. 4. The decoder 400 isconfigured to receive a one-bit flag 220 for each associated referencepicture in a buffer description for the current picture, and the decoder400 comprises a processor 402 configured to decode the one-bit flag 220which explicitly indicates to the decoder whether or not the referencepicture should be included in a list of reference pictures 230. Further,the processor 402 is configured to use the one-bit flag to determine ifthe reference picture should be included in the list of referencepictures.

As illustrated in FIG. 4, the buffer description 210, also referred toas reference picture set contains reference pictures ref pic1, ref pic 2etc and for each reference picture there is a respective one-bit flagflag 1, flag 2 etc which explicitly indicates whether or not thereference pictures should be included in the reference picture list 230.

Further according to an embodiment, the decoder comprises a processor402 configured to interpret that one value of the one-bit flagexplicitly indicates that one reference picture should not be includedin the reference picture list. The processor 402 may also be configuredto interpret that one value of the one-bit flag explicitly indicatesthat one reference picture should be included in the reference picturelist.

According to another embodiment, wherein each picture belongs to a layeridentified by a layer identity, the processor 402 is configured tointerpret that the one-bit flag set to “1” explicitly indicates that onereference picture should be included in the reference picture list andthe one-bit flag set to “0” explicitly indicates that one referencepicture should not be included in the reference picture list.

According to further embodiments, the processor 402 is configured todecode the one-bit flag in a slice header or in a parameter set.

As mentioned in the embodiments described above, the reference pictureswhich have a layer identity higher than the layer identity of thecurrent picture should never be included in the list of referencepictures. That implies that, for missing pictures, the one-bit flag alsocan be used by the decoder to determine if a picture is unintentionallylost.

A procedure in the decoder when the flag is used for each picture in theBuffer Description to verify that no picture is unintentionally lost isdescribed below. Thus, a reference picture list is created, includingthe step of checking the value of the one-bit flag in order to concludeif the reference picture shall be included in the reference picture listor not. For missing pictures, the one-bit flag can also be used todetermine if the picture has been unintentionally lost. One value of theone-bit flag indicates that the reference picture have the same or lowerlayer identity as the current picture which indicates that the missingreference picture is unintentionally lost. Another value of the one-bitflag indicates that the picture may have been correctly removed, i.e.when the missing reference picture has higher layer identity than thecurrent picture. The rest of the decoding process including referencepicture list construction is not affected.

With further reference to FIG. 4, according to a fifth aspect ofembodiments of the present invention a decoder 400 for decoding arepresentation of a current picture of a video stream of multiplepictures using reference pictures is provided. The decoder 400 isconfigured to decode a one-bit flag 220 associated with a referencepicture in a buffer description for the current picture. The decoder 400comprises a processor configured to check a value of the one-bit flagfor each reference picture in the buffer description where thecorresponding reference picture in a decoded picture buffer is missingand if the value of the one-bit flag is indicating that the referencepicture is included in the reference picture list, the processor isconfigured to conclude that the reference picture is unintentionallylost.

Hence a decoder may be configured to perform the following steps todecode a picture according to embodiments:

-   -   The slice header is decoded.    -   A buffer description that includes the flag for each reference        picture in the buffer description is decoded from syntax        elements in the slice header and possibly from an active        Parameter Set such as Picture parameter set or Sequence        parameter set.    -   Picture marking is performed using the buffer description.    -   For each picture in the buffer description the flag is in this        embodiment used to verify that no picture is unintentionally        lost.    -   A reference picture list is created, including the step of        comparing the temporal_id of reference pictures with temporal_id        of the current picture in order to conclude if the reference        picture shall be included in the reference picture list or not.        Accordingly, the flag is used to create the reference picture        list by determining which reference pictures that should be        included in the reference picture list based on the value of the        flag.    -   The picture is decoded.

According to a further alternative, the reference picture listconstruction process is changed so that the layer identity, exemplifiedby the temporal_id of the reference picture is no longer compared to thetemporal_id of the current picture to decide if the picture is includedin the reference picture list(s) or not. It should be noted thatreference pictures having a temporal_id higher than the temporal_id ofthe current picture is never allowed to be included in the list ofreference picture list. Instead said one-bit flag indicates whether thepicture has been unintentionally lost or that the picture has beencorrectly removed is used so that the reference picture is included onlyif the value of the flag is set to that which represents that thereference picture belongs to the same temporal layer as the currentpicture or lower.

With further reference to FIG. 3, an encoder 300 for encoding arepresentation of a current picture of a video stream of multiplepictures using reference pictures is provided. The encoder is configuredto encode a one-bit flag 220 associated with a reference picture in abuffer description 210 for the current picture. The encoder comprises aprocessor 302 configured to set a one-bit flag according to a comparisonbetween the layer identity of each reference picture and the layeridentity of the current picture such that the flag indicates that areference picture is to be included in the reference picture list if thelayer identity of that reference picture is equal to or lower than thelayer identity of the current picture and such that the flag indicatesthat the reference picture is excluded from the reference picture listif the layer identity of the reference picture is higher than the layeridentity of the current picture.

Hence an encoder may be configured to perform the following steps toencode a picture according to embodiments:

-   -   A buffer description is created setting the value of the flag        for each picture in the buffer description according to the        evaluation of comparing layer identity of the current picture        and the reference picture in the buffer description. For        pictures with higher layer identities than the current picture,        each corresponding flag is set to indicate that the pictures        will not be included in the reference picture list. For pictures        with equal or lower layer identities than the current picture,        each corresponding flag is set to indicate that the pictures        will be included in the reference picture list.    -   The buffer description is encoded.    -   The picture is encoded.

The encoder 300 may be a High Efficiency Video Coding, (HEVC) encodere.g. implemented in a mobile terminal. Moreover, the decoder 400 mayalso be a High Efficiency Video Coding, (HEVC) decoder e.g. implementedin a mobile terminal. It should however be noted that the embodimentsare not limited to HEVC.

The processors 302,402 are capable of executing software instructionscontained in a computer program stored in a computer program producte.g. in the form of the memories 301,401. The respective computerprogram product can be a memory being any combination of random accessmemory (RAM) and read only memory (ROM). The respective memory comprisespersistent storage, which, for example, can be any single one orcombination of magnetic memory, optical memory, solid state memory oreven remotely mounted memory.

What is claimed is:
 1. An encoder comprising a processor, wherein theprocessor is configured to: encode a representation of a current pictureof a video stream of multiple pictures; encode, for each of a pluralityof reference pictures included in a buffer description for the currentpicture, a respective one-bit flag according to one of two availablevalues for the one-bit flag, the two available values for the one-bitflag comprising: a first value explicitly indicating to a decoder toinclude the reference picture in a reference picture list for decodingthe current picture; and a second value explicitly indicating to thedecoder not to include the reference picture in the reference picturelist for decoding the current picture; output the representation of thecurrent picture and the one-bit flags, wherein to output the one-bitflags, the processor is configured to indicate to the decoder that areference picture, corresponding to a one-bit flag set to the secondvalue and missing from a decoded picture buffer of the decoder, has beencorrectly removed from the decoded picture buffer.
 2. The encoder ofclaim 1, wherein to encode the one-bit flags, the processor isconfigured to set the one-bit flag corresponding to a first referencepicture of the plurality of reference pictures to the second valueresponsive to determining that a layer identity of the first referencepicture is higher than a layer identity of the current picture.
 3. Theencoder of claim 1, wherein to encode the one-bit flags, the processoris configured to set the one-bit flag corresponding to a secondreference picture of the plurality of reference pictures to the firstvalue responsive to determining that a layer identity of the secondreference picture is not higher than a layer identity of the currentpicture.
 4. The encoder of claim 3, wherein the processor is configuredto set the one-bit flag corresponding to the second reference picture tothe second value in further response to the second reference picturehaving a quantization parameter higher than a given threshold.
 5. Theencoder of claim 1, wherein the first value and second value are “1” and“0,” respectively.
 6. The encoder of claim 1, wherein to output therepresentation of the current picture and the one-bit flags, theprocessor is configured to output the one-bit flags in a slice header ofthe representation of the current picture.
 7. The encoder of claim 1,wherein to output the representation of the current picture and theone-bit flags, the processor is configured to output the one-bit flagsin a sequence parameter set external from the representation of thecurrent picture.
 8. The encoder of claim 1, wherein to output therepresentation of the current picture and the one-bit flags, theprocessor is configured to output the one-bit flags in a pictureparameter set external from the representation of the current picture.9. The encoder of claim 1, wherein to output the one-bit flags, theprocessor is configured to indicate to the decoder that a referencepicture, corresponding to a one-bit flag set to the first value andmissing from a decoded picture buffer of the decoder, has beenunintentionally lost.
 10. A method of picture encoding, implemented byan encoder, the method comprising: encoding a representation of acurrent picture of a video stream of multiple pictures; encoding, foreach of a plurality of reference pictures included in a bufferdescription for the current picture, a respective one-bit flag accordingto one of two available values for the one-bit flag, the two availablevalues for the one-bit flag comprising: a first value explicitlyindicating to a decoder to include the reference picture in a referencepicture list for decoding the current picture; and a second valueexplicitly indicating to the decoder not to include the referencepicture in the reference picture list for decoding the current picture;outputting the representation of the current picture and the one-bitflags, wherein outputting the one-bit flags comprises indicating to thedecoder that a reference picture, corresponding to a one-bit flag set tothe second value and missing from a decoded picture buffer of thedecoder, has been correctly removed from the decoded picture buffer. 11.The method of claim 10, wherein encoding the one-bit flags comprisessetting the one-bit flag corresponding to a first reference picture ofthe plurality of reference pictures to the second value responsive todetermining that a layer identity of the first reference picture ishigher than a layer identity of the current picture.
 12. The method ofclaim 10, wherein encoding the one-bit flags comprises setting theone-bit flag corresponding to a second reference picture of theplurality of reference pictures to the first value responsive todetermining that a layer identity of the second reference picture is nothigher than a layer identity of the current picture.
 13. The method ofclaim 12, wherein setting the one-bit flag corresponding to the secondreference picture to the second value is in further response to thesecond reference picture having a quantization parameter higher than agiven threshold.
 14. The method of claim 10, wherein the first value andsecond value are “1” and “0,” respectively.
 15. The method of claim 10,wherein outputting the representation of the current picture and theone-bit flags comprises outputting the one-bit flags in a slice headerof the representation of the current picture.
 16. The method of claim10, wherein outputting the representation of the current picture and theone-bit flags comprises outputting the one-bit flags in a sequenceparameter set external from the representation of the current picture.17. The method of claim 10, wherein outputting the representation of thecurrent picture and the one-bit flags comprises outputting the one-bitflags in a picture parameter set external from the representation of thecurrent picture.
 18. The method of claim 10, wherein outputting theone-bit flags comprises indicating to the decoder that a referencepicture, corresponding to a one-bit flag set to the first value andmissing from a decoded picture buffer of the decoder, has beenunintentionally lost.
 19. A non-transitory computer readable mediumstoring a computer program product for controlling a programmableencoder, the computer program product comprising software instructionsthat, when run by the programmable encoder, cause the programmableencoder to: encode a representation of a current picture of a videostream of multiple pictures; encode, for each of a plurality ofreference pictures included in a buffer description for the currentpicture, a respective one-bit flag according to one of two availablevalues for the one-bit flag, the two available values for the one-bitflag comprising: a first value explicitly indicating to a decoder toinclude the reference picture in a reference picture list for decodingthe current picture; and a second value explicitly indicating to thedecoder not to include the reference picture in the reference picturelist for decoding the current picture; output the representation of thecurrent picture and the one-bit flags, wherein to output the one-bitflags comprises indicating to the decoder that a reference picture,corresponding to a one-bit flag set to the second value and missing froma decoded pircture buffer of the decoder, has been correctly removedfrom the decoded picture buffer.